1. Field of the Invention
The present invention relates to an acceleration sensor, and more particularly to an acceleration sensor for detecting an acceleration by transforming oscillation levels into electrical signals.
2. Description of the Related Art
In general, the acceleration sensor now in use includes various types such as an electro magnetic type, a piezoelectric element type, and a semiconductor type which are known as detecting an acceleration applied thereto. Among these types of acceleration sensor, the piezoelectric element type of the acceleration sensor has a piezoelectric element deformable in response to the acceleration to detect the acceleration. These piezoelectric element types of the acceleration sensor are applied to automotive vehicles and used for controlling knocking of engine and air bag.
A conventional piezoelectric element type of the acceleration sensor herein raised for example is shown in FIG. 25 to comprise an oscillation plate having a central portion fixed. This type is called “the center-fixed type of acceleration sensor”, i.e., the first conventional acceleration sensor. This center-fixed type of acceleration sensor 100 comprises a fixed metal case 101 having a central bottom portion from which projects a supporting protrusion 101a integrally formed with the central portion. Onto the supporting protrusion 101a is welded and securely connected an oscillation plate 102 made of a metal and in the form of a thin disc shape to facilitate resonance motion of the oscillation plate 102 as shown in FIG. 26. On the upper surface of the oscillation plate 102 is mounted a piezoelectric element 103 in a doughnut shape in a manner that the piezoelectric element 103 is held in axial alignment with the oscillation plate 102. The piezoelectric element 103 has upper and lower surfaces on which are respectively mounted a pair of electrodes 104 axially aligned with the piezoelectric element 103. One of the electrodes 104 is electrically connected with the oscillation plate 102, while the other one of the electrodes 104 is soldered at 105a and thus electrically connected with a metal wire 105 by way of, for instance, wire bonding. The acceleration sensor 100 further comprises an output terminal 107 having one end electrically connected with the metal wire 105 and the other end electrically connected with an exterior connector, not shown, and a cover member 106 in the form of a bowl shape and made of a resin material. The fixed case 101 and the cover member 106 have peripheral edge portions 101c and 106c, respectively, which are firmly coupled with each other to define a closed space 109 having the oscillation plate 102 and the piezoelectric element 103 received therein. Between the peripheral edge portions 101c of the fixed case 101 and 106c of the cover member 106 is disposed an O-ring which serves to hermetically seal the closed space 109.
Another conventional piezoelectric element type of the acceleration sensor herein raised for example, i.e., the second conventional acceleration sensor is shown in FIG. 27. The acceleration sensor 110 comprises a fixed case 111 made of a metal and having a peripheral ledge portion 111c, and a metal base member 112 in the form of a disc shape and also having a peripheral edge portion 112c. The metal base member 112 is welded to and thus securely mounted on the fixed case 111 with the peripheral ledge portion 111c being in registry with the peripheral edge portion 112c so that the fixed case 111 is covered and closed by the metal base member 112. On the metal base member 112 is mounted a connector member 116 also in the form of a disc shape and having a peripheral edge portion 116c fixedly engaged with the peripheral ledge portion 111c of the fixed case 111. The connector member 116 has an output terminal 107 securely mounted thereon and is electrically connected with an exterior connector, not shown. The fixed case 111, the metal base member 112 and the connector member 116 collectively define a closed space 109 in which the oscillation plate 102 and the piezoelectric element 103 are accommodated. The metal base member 112 has a central portion from which downwardly extends a protrusion 112a having the oscillation plate 102 supported thereon, compared with the protrusion 101a of the case base 101 upwardly projected and having the oscillation plate 102 supported thereon as shown in FIG. 25. Both of the oscillation plate 102 and the piezoelectric element 103 are in the form of a doughnut shape and securely supported by the protrusion 112a of the metal base member 112 to ensure that the oscillation plate 102 is oscillatable with respect to the fixed case 111. The connector member 116 is made of a resin material and serves to electrically insulate the metal base member 112 from the fixed case 111. The output terminal 107 securely mounted on the connector member 116 extends through the protrusion 112a of the metal base member 112 and has a lower end electrically connected with one of the electrodes 104 on the piezoelectric element 103 by way of a connecting disc plate 115 soldered at 115a to the lower end of the output terminal 107 and one of the electrode 104. The acceleration sensor 110 comprises an O-ring 118 disposed between the inner peripheral face of the fixed case 111 and the outer peripheral face of the metal base member 112 to hermetically seal the closed space 109. It is preferable that the connecting disc plate 115 has a rigidity as small as possible so that the oscillation plate 102 and the piezoelectric element 103 are not prevented from being oscillated. The connecting disc plate 115 may be replaced with a metal wire having one end electrically connected to the output terminal 107 and the other end electrically connected to one of the electrode 104 on the piezoelectric element 103 in a manner that the oscillation plate 102 is welded on the protrusion 112a of the metal base member 112.
The first and second conventional acceleration sensors 100 and 110 respectively have lower portions formed with male screws 101b and 111b each screwed in to an oscillation object such as an automotive engine or the like to ensure that the oscillation plate 102 is oscillated with respect to the fixed cases 101 and 111 when the oscillation object is oscillated for some reason. The oscillation of the oscillation plate 102 causes the piezoelectric elements 103 to be deformed and energized to generate voltage levels which are outputted to the output terminal 107 through one of the electrodes 104 with the fixed case 101 or 111 and the metal base member 112 grounded.
In general, the piezoelectric element 103 has a capacity C between the electrodes 104 which can produce an electric charge Q when the oscillation plate 102 is oscillated and deformed to produce a stress deformation in the piezoelectric element 103 by exterior oscillations, i.e. accelerations. The electric charge Q thus caused by the stress deformation of the oscillation plate 102 can be measured as voltage V that is represented by the following equation:V=Q/C
It is considered that the oscillation plate 102 has the maximum oscillation amplitude at around its outer peripheral end while the piezoelectric element 103 has the maximum stress deformation value at around its central portion, resulting from the fact that the piezoelectric element 103 is extended and contracted.
The acceleration sensor 100 or 110 has a frequency characteristic under a predetermined level of oscillation corresponding to a predetermined level of acceleration as shown in FIG. 28. FIG. 28 indicates that the output voltage V0 is high at a frequency of the resonance point f0, hereinlater referred to as “resonance frequency f0”, while being flat and low at frequency points in other areas such as medium and low frequency areas. In view of this frequency characteristic, acceleration sensors such as the acceleration sensors 100 and 110 are classified into two different types consisting of a non-resonance type of using a flat portion of the frequency characteristics within a predetermined range of effective frequencies which does not include the resonance frequency f0 and a resonance type of using frequency characteristics having the resonance frequency f0 within a predetermined range of effective frequencies. The acceleration sensors 100 and 110 are adapted to operate with the oscillation plate 102 oscillated at a desired frequency level within a predetermined range of effective frequencies having the upper limit in the vicinity of the resonance frequency f0.
The resonance frequency f0 of the oscillation plate 102 in the form of a disc shape and securely mounted on the central portion of the fixed case can be represented by the following equation (1).
[Eq. 1]fo=α(t/R2)×√{square root over (E/ρ(1−σ2))}  equation (1)
where α=0.172 (constant), t stands for thickness, R stands for radius, E stands for Young's modulus, ρ stands for density, and σ stands for Poisson's ratio.
In the event that the oscillation plate 102 is made of nickel steel, the above parameters are as follows.
t=0.4 (mm), R=7 (mm), E=2×1011 (N/m2), ρ=7.8×103 (kg/m3),
and σ=0.28.
The above parameters render the resonance frequency f0 to be 7.41 (kHz). The resonance frequency f0 is determined primarily by the oscillation plate 102, however, should be decided in consideration of other neighboring elements such as the fixed case 101 and piezoelectric element 103. This is because of the fact that those elements slightly affect the oscillation of the oscillation plate 102.
In order to secure a desired resonance frequency f0 in view of the above fact, the thickness t and the radius R are, in general, required to be appropriately selected for designing the acceleration sensor. In particular, the resonance frequency f0 is affected largely by the radius R as will be seen from the fact based on experimental results that the resonance frequency f0 is varied by a rate of about 1 to 2% as the radius R of the oscillation plate 102 is varied by 0.1 mm with the thickness t unchanged. In the light of the sensitivity of the acceleration sensor, it is evident through repeated experiments that the acceleration sensor 110 shown in FIG. 27 can be produced with sensitivity higher than that of the acceleration sensor 100 shown in FIG. 25. The reason is considered to be due to the fact that the oscillation plate 102 is mounted on the metal base member 112, with the result that the metal base member 112 being not completely rigid is slightly oscillated together with the oscillation plate 102 when it receives acceleration, thereby making it possible for the oscillation of the oscillation plate 102 to be amplified by the metal base member 112.
The electrodes 104 mounted on the piezoelectric element 103 may be categorized into two different groups consisting of a first group of excitation electrodes which is constituted by a pair of electrodes with a small diameter and a second group of detection electrodes which is constituted by a pair of electrodes with a large diameter, and both the first group of the exciting electrodes and the second group of the detection electrodes are coaxially aligned with the piezoelectric element 103. Alternating current is applied to the piezoelectric element 103 through the excitation electrodes thus constructed so as to oscillate the oscillation plate 102 by way of the piezoelectric effect, and energize the detection electrodes, thereby making it possible to measure output voltage through the detection electrodes for carrying out the self diagnostics such as performance and failure diagnostics, or the calibration of the acceleration sensor. In the conventional acceleration sensors 100 and 110, the oscillation plate 102 is supported by the supporting protrusion 101a and the protrusion 112a, respectively. There are, however, provided many variations of the acceleration sensor. The oscillation plate may be in the form of a disc shape having a peripheral portion clamped, or in the form of a rod having one end securely mounted. The fixed cases 101 and 111 are classified into two types consisting of one-terminal type of having the fixed case serve as a ground and two-terminal type having two terminals, one of which serves as a ground.
FIG. 29 shows a third conventional acceleration sensor 120 of the piezoelectric element type and the non-resonance type comprising a piezoelectric element and a weight. This type is called “the compression type of the acceleration sensor”. The acceleration sensor 120 comprises a connector body 126 and a fixed case 121. The connector body 126 has a peripheral edge portion. The fixed case 121 is made of a metal material and has an open peripheral end portion 121c which is bent to form a fitting portion fittingly connected with the peripheral edge portion of the connector body 126 to define a closed space 109 having a weight 122 and a piezoelectric element 123 received therein. The connector body 126 has a terminal 107 mounted thereon. The piezoelectric element 123 is in the form of a doughnut shape and has upper and lower surfaces on which are respectively mounted a pair of detection electrodes 124 consisting of a first electrode and a second electrode 124a and 124b. The weight 122 is made of a metal material and in the form of a cylindrical shape. The weight 122 is held in contact with the first detection electrodes 124a on the upper surface of the piezoelectric element 123 as shown in FIG. 30. The terminal 107 is adapted to be electrically connected to the first electrode 124a of the piezoelectric element 123 and an exterior connector, not shown. The weight 122 is securely mounted on the piezoelectric element 123 by means of a fastening screw 125 to pressurize the piezoelectric element 123 toward the center bottom portion of the fixed case member 121. The fastening screw 125 is screwed in through a screw hole 121d formed in the center bottom portion of the fixed case 121.
The second detection electrodes 124b forming part of the acceleration sensor 120 is mounted on the lower surface of the piezoelectric element 123 to be electrically connected with the fixed case 121 while the first detection electrodes 124a is mounted on the upper surface of the piezoelectric element 123 to be electrically connected with the weight 122 and a contact terminal 127. The contact terminal 127 is in the form of a L-shape and securely mounted on the weight 122 by the fastening screw 125. The contact terminal 127 is electrically connected with the output terminal 107 of the connector body 126 through a wire 129 having both ends 129a and 129b soldered with the contact terminal 127 and the output terminal 107, respectively. The acceleration sensor 120 further comprises an insulation tube 125a and an insulation spacer 125b interposed between the weight 122, the piezoelectric element 123, and the fastening screw 125 to prevent the fixed case 121 and the output terminal 107 from forming a short circuit. The acceleration sensor 120 further comprises an O-ring 128 disposed between the open peripheral end portion 121c of the fixed case 121 and the peripheral end portion of the connector body 126 to hermetically seal the closed space 109 in which electrical components such as the piezoelectric element 123 are accommodated.
The acceleration sensor 120 thus constructed makes it possible to use the fixed case 121 as a ground for an electric circuit, and output an output voltage of the piezoelectric element 123 through the weight 122 and the output terminal 107. The fixed case 121 has a bottom portion formed with a male screw 121b fixed to an exterior object such as an engine, not shown, to be detected for an acceleration. An oscillation caused by the exterior object is transmitted to the weight 122, which exerts a load (compression force) on the piezoelectric element 123 in response. The piezoelectric element 123 generates an output voltage indicative of the acceleration and outputs the output voltage through the output terminal 107. The acceleration is thus detected on the basis of the output voltage received from the output terminal 107. The acceleration sensor 120 has a frequency characteristic similar to that of the aforesaid acceleration sensors 100 and 110 under a predetermined constant level of oscillation, i.e., constant acceleration as shown in FIG. 28. The resonance frequency f0, however, does not appear to a recognizable extent depending upon the condition of the acceleration sensor assembled with other devices and machines. This results from the fact that the resonance frequency f0 moves to a higher frequency range due to the fact the fastening screw 125 is screwed in through the central portion of the piezoelectric element 123 and the weight 122 with a relatively small screwing force exerted on the peripheral portion of the acceleration sensor 120, thereby causing the acceleration sensor 120 to be resonantly oscillated in a high frequency range. This means that the fastening screw 125 is required to be produced with high precisions for torque and machining of the engagement faces of the fastening screw 125.
The acceleration sensor 120 of such non-resonance frequency type is usually designed to be oscillatable with the resonance frequency f0 of 20 kHz or greater, which is out of the range of effective oscillation frequencies, so that the flat portion, i.e., V0 of the output voltage range is actually used for detecting an acceleration (see FIG. 28). V0 also stands for “the sensitivity” of the acceleration sensor. The basic principle of the acceleration sensor 120 is that an acceleration [G] exerted on a weight 122 of mass [m] causes a stress strain [F] on the piezoelectric element 123 to generate an output voltage V0 indicative of the acceleration in accordance with the equation as follows.F=m·G V0≈α·F·t/Swhere α stands for a constant such as piezoelectric constant, S stands for the area of the detecting electrode 124 of the piezoelectric element 123, and t stands for the thickness of the piezoelectric element 123.
As will be understood from the foregoing description, the methods to enhance the sensitivity of the acceleration sensor 120 is considered to include:    (1) an increased weight of the weight 122, and/or    (2) an increased factor “t/S” of the piezoelectric element 123. (The increase in the factor “t/S”, however, is limited to a predetermined level decided based on its size and volume requested.)
It is therefore understood that the size, especially, the height of the acceleration sensor is required to be enlarged in order to enhance the sensitivity.
The acceleration sensor 120 may comprise a gold plated connecting terminal in place of a lead line such as the wire 129 having the output terminal 107 electrically connected with the weight 122 (the contact terminal 127). The acceleration sensor 120 is not limited to the one-terminal type of having the fixed case 121 serve as a ground but also includes the two-terminal type having two terminals, one of which serves as a ground. The electrodes 124a and 124b of the piezoelectric element 123 may be divided into two groups consisting of the first group of electrodes serving for detecting an acceleration and the second group of electrodes serving for performing the self diagnostics or calibration.
As will be seen from the forgoing description, the first conventional acceleration sensor 100, however, encounters such problems that it is difficult to automatically assemble the acceleration sensor 100 resulting from the fact that one of the electrodes 104 of the piezoelectric element 103 is required to be electrically connected with the output terminal 107 of the cover member 106 through the wire 105 having both ends soldered with them, respectively, by way of, for instance, wire bonding. This leads to the fact that the production cost of the acceleration sensor 100 rises.
As will be seen from the foregoing description, the second conventional acceleration sensor 110 requires no wire connections, thus makes it possible to automatically assemble the acceleration sensor 110 and improve the sensitivity in comparison with the first conventional acceleration sensor 100. The second conventional acceleration sensor 110, however, encounters another problem that oscillation in a high frequency range beyond 10 kHz is easily transmitted through constitutional parts and elements within the acceleration sensor 110 such as the fixed case 111, and the oscillation thus transmitted affects the characteristics of the acceleration sensor 110 such as phase characteristics. The second conventional acceleration sensor 110 also encounters another problem that the metal base member 112 is not perfectly rigid but could be slightly distorted and loosened due to temperature degradation resulting from the fact that the connector member 116 has a peripheral edge portion fixedly engaged with the peripheral ledge portion 111c of the fixed case 111, and a gap between the fixed case 111, the metal base member 112, and the connector member 116 is subject to vary at an elevated temperature. An oscillation noise generated from the output terminal 107 is transmitted to the connector member 116. The metal base member 112 thus distorted and loosened will transmit the oscillation noise to the oscillation plate 102, thereby deteriorating the accuracy of the acceleration sensor 110 for detecting an acceleration.
Furthermore, the first and second acceleration sensors 100 and 110 encounter another problem. As a result of an analysis by means of the finite element method, the oscillation plate of acceleration sensors of the center-fixed type such as the acceleration sensors 100 and 110 is oscillatable in three different modes consisting of a 1/1 oscillation mode, a 1/2 oscillation mode, and a 1/4 oscillation mode as shown in FIG. 31.
FIG. 31A shows the oscillation plate oscillating in the 1/1 oscillation mode where the oscillation plate is irregularly deformed to have the peripheral portion oscillated with a single vector in the oscillation direction of the oscillation plate when the oscillation plate is oscillated with respect to the fixed case member at a resonance frequency f0, FIG. 31B shows the oscillating plate oscillating in the 1/2 oscillation mode where the oscillation plate is irregularly deformed to have two different half parts of the peripheral portion oscillated with their respective different vectors opposite to each other in the oscillation direction of the oscillation plate when the oscillation plate is oscillated with respect to the fixed case member at a first noise frequency f01, and FIG. 31C shows the oscillating plate oscillating in the 1/4 oscillation mode where the oscillation plate is irregularly deformed to have four different parts of the peripheral portion oscillated with their respective different vectors opposite to one another in the oscillation direction of the oscillation plate when the oscillation plate is oscillated with respect to the fixed case member at a second noise frequency f02. The first noise frequency f01 is approximately half of the resonance frequency f0, and the second noise frequency f02 is in the vicinity of the resonance frequency f0. The oscillation of the oscillation plate in the 1/2 or 1/4 oscillation mode does not cause any problem as long as the oscillation plate has two or four different parts of the peripheral portion evenly oscillated with respective vectors opposite to one another in the oscillation direction of he oscillation plate, and the output voltage thus generated is counterbalanced. The oscillation plate, however, could have two or four different parts of the peripheral portion unevenly oscillated with respective vectors opposite. The uneven oscillation of the oscillation plate causes the piezoelectric element to generate a noise output voltage and deteriorate the accuracy of the acceleration sensor. Especially the oscillation of the oscillation plate in the 1/2 oscillation mode causes noise output voltage, hereinlater referred to as “spurious”. This leads to the fact that the oscillation of the oscillation plate at a frequency in the vicinity of the first noise frequency f01 causes an error in detecting an acceleration.
The oscillation plate used for the acceleration sensor of the non-resonance type is thick. It is considered that the weight balance of the oscillation plate with respect to the support portion affects the quality of the acceleration sensor.
As will be seen from the foregoing description, the third conventional acceleration sensor 120 encounters a problem that it is difficult to automatically assemble the acceleration sensor 120, and thus the production cost of the acceleration sensor 120 rises resulting from the fact that the acceleration sensor 120 has many parts and is complex in construction. The third conventional acceleration sensor 120 also encounters another problem that the acceleration sensor 120 is required to be produced with high precision for torque and machining of the engagement faces of the fastening screw 125 resulting from the fact that the fastening screw 125 is screwed in through the central portion of the weight 122 and the piezoelectric element 123 so that the weight 122 and the piezoelectric element 123 are tightly held in contact with each other toward the bottom surface of the fixed case 121. This further leads to another problem that the size (especially, the height) of the acceleration sensor 120 is required to be enlarged and the production cost is increased. The acceleration sensor 120, furthermore, encounters another problem that the fastening screw 125 may be loosened, thereby causing the acceleration sensor 120 to deteriorate the accuracy for detecting an acceleration.